Under what conditions is $\lim_{x\to c} f(x)=e^{\lim_{x \to c} \ln(f(x))}$?
I saw this limit in an article used to show that:
$$\lim_{\rho \to 0} [\alpha x_{1}^{\rho} + (1-\alpha) x_{2}^{\rho}]^{\frac{1}{\rho}} = x_{1}^{\alpha}x_{2}^{1-\alpha}$$
where the "trick" is used to apply l'hopitals rule to:
$$\lim_{\rho \to 0} \frac{\ln(\alpha x_{1}^{\rho} + (1-\alpha) x_{2}^{\rho})}{\rho} $$
However I was unaware that this trick existed before today, and I am wondering if there are conditions under which it applies?